DR KEITH WHEELER / SCIENCE PHOTO LIBRARY DR KEITH WHEELER / SCIENCE PHOTO LIBRARY
Lilac stem. Light micrograph of a transverse section through the young woody stem of a lilac (Fraxinus excelsior) tree. The thick epidermis (solid green) is being sloughed (pushed/broken) off by developing underlying cork layers (large radial cells, green), under this is a layer of collenchyma (green). The lower cortex has primary phloem fibres (brownish-black) above a ring of phloem sieve tubes (green). Under the phloem is a ring of cambium (green) above a thick ring of wood (xylem, also green), composed of xylem vessels (large circular holes) and tracheid cells. The medullary rays (radial lines, brownish-green) transport water and minerals to the outside of the stem. The solid pith (centre) is composed of large parenchyma cells (light-green). Magnification: x14 when printed 10 centimetres wide.
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